Elderly people often suffer from taste disorder, with subsequent appetite loss. A reduction in umami-taste
sensitivity is strongly related to appetite loss. Under these circumstances, how is umami taste considered and
applied to foods for the elderly in Japan? In this study, we measured the concentration of L-glutamate (a typical
umami amino acid) in 658 different miso soups served in 220 institutions for the elderly in Japan. Miso soup is a
popular Japanese savory soup made with soup stock (dashi) and fermented soybean paste (miso), both of which
are abundant in umami substances. The concentration of L-glutamate in the miso soups was the highest among the
measured amino acids (Av ± SD: 156.3 ± 101.3 mg/100 g) with a very large range (15.7–697 mg/100 g, CV: 64.8%).
We also measured the sodium concentration in the same miso soups (Av ± SD: 328.5 ± 70.1 mg/100 g) and found
that it is controlled in a narrow range (CV: 21%). We should further investigate whether these concentrations are
preferred by the elderly, for the improvement of their appetite and nutritional status.

Keywords

Elderly; miso soup; Umami taste; Glutamate
concentration

Introduction

Mealtimes are often a great pleasure in daily life, especially for
the elderly [1]. However, many elderly persons suffer from taste and/
or smell impairment caused by senescence itself, medication, and/or
diseases, and they are thus unable to enjoy many foods [2,3]. According
to the research on the taste sensitivity of patients with taste impairments,
a reduction in umami taste sensitivity showed a strong association with
the reduction of food palatability [4,5]. Umami taste-which is one of
the five basic tastes along with sweet, salty, sour and bitter tastes-is a
major taste quality of soup stock, the so-called “savory” taste. The most
typical umami substance is L-glutamate (Glu), a non-essential amino
acid [6]. In the present study, we investigated the Glu concentration
in miso soup served in hospitals and nursing homes for the elderly in
Japan. miso soup is a very popular Japanese traditional savory soup
and is served at least once a day in such institutions. miso soup is
made by boiling various ingredients such as vegetables in soup stock
(dashi) and seasoning the stock with fermented soybean paste (miso).
We collected miso soup from institutions for the elderly all over Japan,
because there are many local varieties of dashi and miso that are used as
the source of umami substances for miso soup. In addition to the Glu
concentration in miso soup, we analyzed the concentrations of Inosine
5’-Monophosphate (IMP) [7] and Guanosine 5’-Monophosphate
(GMP) [8], which are other types of umami substance, nucleotides. The
sodium (Na) concentrations of the soup samples were also analyzed.

Materials and Methods

Sample collection

Ten miso soups were collected from a typical elderly hospital in
Kyushu, a southern island in Japan (Institution A), and three miso soups
were collected from a typical elderly nursing home in the same area
(Institution B). We also collected 2 or 3 miso soups from each of the five
institutions from each prefecture in Japan. The miso soup samples were
frozen immediately after being prepared at the originating institution
and thawed just before our analysis.

Analysis of Glu concentration

The chunky ingredients in each miso soup sample were removed
with a strainer, and the supernatant of the soup was collected. The supernatant was diluted 20-fold with ultrapure water and then filtrated
using a 0.45-μm filter and a 10-kDa centrifugal ultrafiltration unit. The
amino acid concentration in the filtrate was analyzed by an L-8900
amino acid analyzer (Hitachi High-Technologies Corp., Japan).

The Na concentration of the supernatant was analyzed by an
ICPS-8100 inductively coupled plasma atomic emission spectrometer
(Shimadzu Corp., Japan).

Statistical analysis

Analysis of variance (ANOVA) and calculation of correlation
coefficients were performed using SPSS 16.0 (SPSS Japan).

Results and Discussion

Variance of Glu concentration in miso soup within institutions
for the elderly

We first analyzed the variation of the Glu concentration of miso
soup within two institutions, using samples from Institutions A and B
(Figure 1). The Glu concentration of miso soup at Institution A was
36 ± 4.0 mg/100 ml (Av ± SD, n=10), and that of Institution B was
98 ± 7.0 mg/100 ml (n=3). Despite the variation in ingredients, the variance of Glu within each institution was relatively small, with an
approx. 10% coefficient of variation (CV) in both institutions. Thus, the
Glu concentration in miso soup might strongly depend on the dashi and miso used, and might depend less on other ingredients. Therefore,
we decided to collect 2 or 3 types of miso soup from each institution
to determine the Glu concentration in the miso soup served in such
institutions for the elderly in Japan. Although every institution for the
elderly serves several special diets (e.g. low-salt diets for patients with
hypertension, low-sugar diets for patients with diabetes), we collected
only miso soups used for patients receiving a regular diet for the
comparison.

Figure 1:Variance of Glu concentration within two institutions. The dotted lines represent the average for institution A and institution B.

Concentrations of umami substances, Glu, and nucleotides in
miso soup served in institutions for the elderly in Japan

We collected 658 samples of miso soup served as part of the
regular patient diet from 220 institutions, which represented about five institutions per prefecture in Japan. The samples’ average concentrations
of amino acids including Glu are shown in Table 1. Glu was the most
abundant protein amino acid in free form; however, its concentration
showed a large CV (Av±SD: 156.3 ± 101.3 mg/100 g (range: 15.7–
697 mg/100 g, CV: 64.8%)).

Table 1:Concentration of amino acids, nucleotides, and Na. Amino acids are
shown in abbreviation of IUPAC.

In the one-way ANOVA, the main effect of institution was
significant (F(219, 438)=6.110 (p=0.000)). The main effects of dashi (five categories: dried bonito (katsuo-bushi), dried sardine (iriko),
ready-made, natural ingredients + ready-made, and non-classifiable
dashi) (F(4, 653)=37.538 (p=0.000)), and miso (seven categories:
soybeans only, soybeans + rice, soybeans + wheat/barley, mixed,
umami seasoning-added, umami seasoning-added + mixed, and nonclassifiable
miso) (F(6, 651)=5.071 (p=0.000)) were significant. That is,
because each institution had its own usage of dashi and miso for miso
soup, the Glu concentration of miso soup differed by institution, but the
Glu concentration was almost constant within each institution, as in the
cases of Institution A and Institution B.

The average Glu concentration by prefecture (Figure 2) was higher
in the middle region of Japan and lower in the southern region of the
country. Most of the institutions in the southern region used dried
sardines for dashi (iriko-dashi) and soybeans and wheat/barley miso
(mugi-miso), which are relatively low in Glu content. In the middle
part of Japan, such as the Kanto district where Tokyo is located, many
institutions used ready-made dashi in which Glu and IMP/GMP are
supplemented as umami seasoning. Several dietitians at institutions
with low-Glu content in high-Glu districts, such as Kanto district,
reported that elderly patients had complained about the unpalatability
of miso soup with a diluted taste. This might be partly because the
elderly people in such high-Glu districts were used in foods with high
Glu contents.

Figure 2:Average Glu concentration by prefecture. The dotted line represents the average.

We also measured the concentration of another type of umami
compound, the 5’-purinemononucleotides IMP and GMP, which
show synergistic umami enhancement when mixed with Glu (Table 1).
These nucleotides are present in meat products such as dried fish
used for dashi, e.g., dried bonito and dried sardines. We found that
the concentration of IMP was 8.8 ± 8.5 mg/100 ml, and that for GMP
was 1.6 ± 3.1 mg/100 ml. However, miso contains phosphatase, which
degrades nucleotides into nucleosides (compounds lacking umami
taste) during cooking and storage, and thus our measurements of the
concentrations of IMP and GMP of miso soup might be less reliable.

Relationship between concentrations of Glu and Na

The Na concentration fell within a narrower range than that of Glu,
between 142 and 596 mg/100 g (328.5 ± 70.1 mg/100 g, CV: 21%), as shown
in Table 1. The relationship between Glu and Na is shown in Figure 3.
A significant positive correlation was found between the Glu and Na
concentrations (r=0.534 (p=0.000)). miso contains high Glu and Na because
it is made with salted and fermented soybeans. Some ready-made dashi contains high Glu and Na because its major ingredients are sometimes NaCl
and umami compounds. These aspects of miso might explain the positive
correlation between the concentrations of Glu and Na in miso soup.

Figure 3:Glu and Na concentration within each institute. The line pattern of each symbol shows the type of dashi, and the shape of each symbol represents the
type of miso, as shown under the graph. One symbol represents an average value (2 or 3 miso soups) of each institution. Therefore, each symbol represents the
major dashi and miso values of each institution. Concentrations are represented in logarithmic axes, because taste intensity is proportionate to the logarithmic
value of the concentration of each tastant.

Umami taste in foods for the elderly

Most people perceive that overall flavor is enhanced when umami
compound is added to foods appropriately. For example, when an umami
compound, Monosodium L-Glutamate (MSG), was added to a diluted
beef broth, the flavor characteristics such as thickness, mouth-fullness,
impact, and overall taste were improved [9]. In another study, umami
compounds were important for the characteristic flavor of fish, crab,
clam, and other seafoods [10,11]. Thus, because umami compounds
intensify flavor, Glu salts are sometimes used for compensation for the
bland taste of salt-reduced foods [12–18]. Several reports showed that salt content could be reduced by as much as 30% by using MSG, though
it contains Na (0.12 g Na/1 g MSG: cf. 0.40 g Na/1 g NaCl) [12,15].

However, the elderly often have appetite loss due to low umamitaste
sensitivity [4], and it may be possible to improve their appetite
by intensifying the umami taste via the fortification of the appropriate
seasonings. We might be able to intensify the savory taste of Japanese
miso soup by partial substitution of miso or ready-made dashi with a
small amount of umami seasonings such as MSG, reducing Na at the
same time to reduce the risk of hypertension. Many Glu-rich traditional
fermented seasonings around the world [19] tend to have high Na
content because NaCl is indispensable for bacteriostasis during longterm
fermentation. In Asia, there are various traditional local high-salt
sauces or pastes made with soybeans, fish, shrimp, and/or meat. Elderly
people tend to prefer traditional dishes cooked with such traditional
fermented seasonings; e.g., the miso soup made with fermented salted
soybean paste (miso) examined in the present study. In such scenarios,
the partial substitution of fermented seasonings by umami substances
might be effective for cooking for the elderly, especially in persons with
reduced umami-taste sensitivity.

Conclusion

We measured the concentration of a typical umami compound,
Glu, of miso soup, a popular Japanese soup served in institutions for the
elderly in Japan. The Glu concentration was the highest among amino
acids (Av ± SD: 156.3 ± 101.3 mg/100 g). However, its range was very
large (15.7–697 mg/100 g, CV: 64.8%), unlike that of the miso soup Na
concentration.

Acknowledgements

We thank the institutions for their supply of miso soup samples and the staff
of Ajinomoto Pharmaceuticals Co., Ltd. and Ajinomoto Nutrition Foods Co., Ltd. for
their support in contacting respective institutions.

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